Which of these three are valid/accepted idioms? Is one of them the "original" one, and others variations on it? Do they have slightly different shades of meaning?
Following from Elian's answer, the use of Google Books trend viewer was excellent, but left me perplexed - "in similar vein" sounded like the more common option to me.
It turns out, the preposition at the end of the phrase is important. If you do the same search without "to" or "as", then you get, from most common to least common:
- "In a similar vein"
- "In the same vein"
- (almost none in comparison to the others) "Along a similar vein"
This aligns closer to my gut feelings. The problem is that when used, these idioms are usually used as a comparison. So for example: "The cat sat on the mat; in a similar vein, the frog sat on the log", or, "His grandma travelled to Africa and truly found herself - and in the same vein, he journeyed to Australia in the hope that he could truly become himself.".
In answer to the question about difference in meaning, I would argue it is negligible. Same is stronger than similar, but the idiom is about finding a commonality between two disparate scenarios, and I don't think there is any objective test that you could do to determine whether same or similar is appropriate to your situation, and I cannot think of a circumstance where one would be wrong and the other right.
I've ignored the third one because I think, in this case, you could make a case for saying that where there is an ongoing transition, "along a similar vein" would be more appropriate. For example. "Jon focussed his investigation on hookers and drug dealers; Betty continued her investigation along a similar vein". In this usage, I imagine you might find that idiom more often close to the end of the sentence.
[IN SINGULAR] A distinctive quality, style, or tendency: (he closes his article in a somewhat humorous vein)
The figurative use of vein with the meaning explained above has been around for quite a while:
Vein, etimology c.1300, from Old French veine "vein, artery, pulse" (12c.), from Latin vena "a blood vessel," also "a water course, a vein of metal, a person's natural ability or interest," of unknown origin. The mining sense is attested in English from late 14c. (Greek phleps "vein" had the same secondary sense). Figurative sense of "strain or intermixture" (of some quality) is recorded from 1560s; that of "a humor or mood, natural tendency" is first recorded 1570s.
The expression you mention are very close in the figurative meaning the in the meaning they convey.As their usage, Ngranshows that the expression *in similar vein'and 'in the same vein' are the most used.